This invention relates generally to improved methods and apparatus for increasing the efficiency of tubing extrusion.
As known to those skilled in the tubing extrusion art, a hollow cylindrical workpiece is advanced over a mandrel and through an extrusion die to produce hollow product of reduced diameter which product, of course, is the tubing.
As is further known to those skilled in the art, with regard to providing relative movement between objects, the friction factor between the two objects is of paramount consideration. Such friction factor, as is further known to those skilled in the art, is defined as the moving force (F.sub.m) required to slide the first object against or over the second object divided by the contact force (F.sub.c) between the objects, which may be expressed as: EQU friction factor (f)=F.sub.m /F.sub.c
As is still further known to those skilled in the art, the friction factor (f) is a dimensionless quantity or number, is a characteristic of the two objects particularly a characteristic of their engaged surfaces, and therefore is constant given the same conditions, particularly the same surface conditions, of the two objects. Hence, for two objects the moving force may be expressed as: EQU F.sub.m =(f)F.sub.c
Therefore the force required to produce relative movement between the two objects may be increased by increasing the contact force F.sub.c and may be decreased by decreasing the contact force F.sub.c. Accordingly, the efficiency of apparatus wherein relative movement between a first object and a second object is desired, and wherein relative movement between the second object and a third object is not desired, may be increased by decreasing the contact force between the first and second objects and by increasing the contact force between the second and third objects. The present invention is based upon this recognition.
More particularly, in the extrusion of tubing, especially the continuous extrusion of tubing, wherein a hollow cylindrical workpiece having inner and outer surfaces is advanced over the outer surface of a stationary cylindrical mandrel and through an annular extrusion die by force applied to the outer surface of the workpiece by an advancing drive member having an inner surface surrounding and in operative engagement with the outer surface of the workpiece to produce a hollow cylindrical product of reduced diameter (i.e. tubing), the efficiency of the extrusion apparatus may be increased by decreasing the moving force, F.sub.m1, that is the force required to produce relative movement between the outer surface of the mandrel and the inner surface of the workpiece and/or by increasing the moving force, F.sub.m2, that is the force required to produce relative movement between the outer surface of the workpiece and the inner surface of the drive member; relative movement between the workpiece and the drive member would, of course, stop advancement of the workpiece which would stop extrusion of the tubing, but by increasing the moving force, F.sub.m2, required to produce relative movement between the workpiece and the drive member, larger or higher driving force can be applied to the workpiece by the drive member without causing relative movement therebetween and hence the efficiency of the tubing extrusion can be increased because higher driving forces applied to the workpiece will cause the workpiece to be advanced more positively over the mandrel and through the extrusion die.
The expression used variously in the following specification and appended claims, "reducing said first moving force (i.e. the moving force, F.sub.m1, required to produce movement between the mandrel and the workpiece) relative to said second moving force (i.e. the moving force, F.sub.m2, required to produce movement between the workpiece and the drive member) to enhance said extrusion process," is used to describe the simultaneous reducing of the first moving force (F.sub.m1) and the increasing of the second moving force (F.sub.m2), and to describe the reducing of the first moving force (F.sub.m1) while the second moving force (F.sub.m2) remains constant or vice versa.
It will be further understood that the expression, "an advancing drive member having an inner surface in operative engagement with the outer surface of the workpiece," is used to describe the condition where the inner surface of the advancing drive member is in direct frictional engagement with the outer surface of the workpiece; the condition where a shear transmitting medium, such as wax or heavy grease, is intermediate the inner surface of the advancing drive member and the outer surface of the workpiece and wherein shear forces are transmitted through the shear transmitting medium to the workpiece to advance and extrude the workpiece; and the condition where there is a low viscosity oil is intermediate the inner surface of the advancing drive member and the outer surface of the workpiece and wherein the drive member is forced into engagement with the workpiece to squeeze out excess low viscosity oil to produce a boundary lubrication condition between the workpiece and the drive member through which boundary lubrication condition the drive member advances the workpiece through the extrusion die.
Accordingly, it will be understood that the object of the present invention is the provision of improved methods of and apparatus for reducing the moving force required to produce relative movement between the mandrel and the workpiece relative to the moving force required to produce relative movement between the workpiece and the drive member whereby the extrusion of tubing is enhanced.